A train starts from rest from 2 stations and traveled with uniform acceleration 1m/s. It travels with uniform velocity for 15secs, the brakes are then applied so that a uniform retardation is obtained and the train comes to rest in a further 5secs. Calculate the total distance travelled

What does this mean: "A train starts from rest from 2 stations?"

For each segment of the train's travel you can apply the formula to find the distance traveled for that segment. However, you have left out some critical information regarding the first segment: you wrote "traveled with uniform acceleration 1m/s," but you need to know either the time of the first phase or the distance covered in order to calculate the velocity at the end of the acceleration phase, which you need in order to calculate the distance of the second segment. Also note that correct units for acceleration is m/s^2, not m/s.